What was the first successful interplanetary mission?

Mariner 2 was the first successful interplanetary mission.

NASA launched Mariner 2 toward Venus on August 27, 1962, using an Atlas-Centaur rocket. It flew past Venus on December 14, 1962, becoming the first spacecraft to conduct a successful planetary encounter and the first to return data from another planet.

The probe measured Venus’s temperature and atmosphere and helped show that the planet was extremely hot. It also made important observations of the solar wind and interplanetary space.

Mariner 2 did not enter orbit around Venus or land there; it performed a flyby. Mariner 4 later became the first spacecraft to fly past Mars and return close-up images of another planet’s surface.